1. Field of the invention
The present invention relates to a method for overpressure detection and pore pressure changes monitoring. More specifically, the present invention relates to a method for remotely detecting compartments with abnormally high pore pressure (overpressure) in subsurface gas, liquid hydrocarbon, or water reservoirs by combining compressional-wave and shear-wave measurements.
2. Description of Related Art
The compressional-wave (P-wave) and shear-wave (S-wave) acoustic data are generally used to obtain information regarding the presence of hydrocarbons in subsurface geological formations, as well as to obtain information concerning such subsurface formation properties as porosity, lithology (i.e., the character of a rock formation), formation fluid, and formation mechanical properties.
Identifying the compressional-wave and measuring its velocity is generally not difficult to carry out. In borehole logging, for example, the compressional-wave is the fastest propagating wave in the formation, it is non-dispersive, and is the first to reach borehole receivers. By measuring the arrival times of these waves at the receivers, the wave velocity near the array of receivers can be determined. In contrast, a shear-wave may have lower amplitude and higher noise level than a compressional-wave. Because
it propagates more slowly through the formation, the shear wave arrives later in time and may be obscured by compressional energy. Identifying a shear-wave and measuring its velocity is considerably more difficult.
When compressional-wave velocity and shear-wave velocity (Vp and Vs, respectively) information is available from field measurements, the ratio of compressional-wave velocity to shear-wave velocity (Vp/Vs) may provide valuable additional information. Prior art techniques have also used the ratio Vp/Vs to identify hydrocarbon-bearing zones and formations.
The traditional method of estimating overpressure in subsurface gas, liquid hydrocarbon, and water reservoirs is to look for abnormally low compressional-wave velocity since low compressional-wave velocity typically corresponds to low differential pressure (where differential pressure is the confining pressure minus the pore pressure) and/or abnormally high porosity. Thus, the effect of compressional-wave velocity decreasing with increasing pore pressure is used to detect for overpressure.
However, methods that use compressional-wave velocity information for overpressure detection are not very accurate or reliable. Compressional-wave velocity does not uniquely indicate pore pressure because compressional-wave velocity also depends, among other factors, on porosity, mineralogy, pore fluid, and texture of the rock. For example, as illustrated in FIGS. 1a and 1b, laboratory experiments on rock data collected from the North Sea and Gulf of Mexico indicate that at the same differential pressure of 20 MPa and in the same porosity range, compressional-wave velocity in gas-saturated Gulf of Mexico sandstones (gray symbols) is smaller than that in dry-room North Sea sandstones (black circles), mostly due to textural differences (see FIG. 1a). At the same time, data indicate that compressional-wave velocity in the Gulf of Mexico sandstones at 20 MPa is about the same as in the overpressured (5 MPa differential pressure) North Sea sandstones (see FIG. 1b). The low compressional-wave velocity in the Gulf of Mexico sandstone group may be mistakenly attributed to overpressure.
More recent methods for overpressure detection have used compressional-wave velocity and density or porosity data, and then employed shear-wave velocity to correct for fluid effects. However, these methods still are not unique and thus not very accurate for compartment overpressure detection because they do not take into account velocity variations due to rock texture and porosity.
Although some methods have used the ratio of compressional-wave to shear-wave velocity (Vp/Vs) or the Poisson""s ratio (an elastic constant based on Vp/Vs ratio) to identify hydrocarbon-bearing zones or to provide an estimate of the character of the rock formation, none of these methods have used the Vp/Vs ratio or the Poisson""s ratio as an overpressure indicator.
It is desirable to develop a reliable and unique method for overpressure detection from compressional-wave and shear-wave data. It is also highly desirable to provide a simple method for remotely detecting compartments with abnormally high pore pressure (overpressure) in subsurface gas, liquid hydrocarbon, or water reservoirs. It is further desirable to provide a method for improving the safety of on-land and offshore drilling operations by detecting or predicting abnormally high pressure in advance of a drilling operation.
The present invention describes a method for identifying overpressure in a subsurface formation, wherein a Poisson""s ratio or a Vp/Vs ratio for the subsurface formation is first determined from compressional-wave measurement and shear-wave measurement. Comparing the determined Poisson""s ratio (or Vp/Vs ratio) value with a plurality of known Poisson""s ratio (or a plurality of known Vp/Vs ratio) values representative of the subsurface formation permits an overpressure to be identified in the subsurface formation.
Additional features and benefits of the present invention will become apparent from the detailed description, figures, and claims set forth below.